The present invention relates in general to cardiovascular diagnosis techniques, and in particular to a new, simple, inexpensive yet accurate system for the diagnosis of arterial stenosis, that is, cholesterol deposits in the carotid artery.
The carotid artery is the artery that carries blood from the heart to the brain. Carotid arterial stenosis is a cerebrovascular disease that is caused by cholesterol deposits inside the artery. These deposits clog the artery and prevent the normal flow of blood. During the systolic cycle, the heart pumps blood into the arterial system, the velocity of blood is high, and the flow of blood is turbulent even if there is no stenosis. If the artery is "clean" there is no turbulence inside the artery during the diastolic cycle when the heart does not pump and when blood flows slowly in the opposite direction due to the elasticity of the arterial system. However, if there are deposits inside the carotid artery (stenosis) the flow of blood will be turbulent even during the diastolic cycle.
The "Doppler frequency-shift effect" was predicted by the astronomer Christian Doppler in 1842 and was experimentally confirmed in 1845. It was 30 years after ultrasound had been used in the military to detect submarines, that the first practical ultrasonic Doppler device for use in medicine was designed and constructed by Satomura in 1956.
Three decades later, there has been a widespread expansion of Doppler ultrasonic applications into virtually all areas of cardiovascular diagnostics.
According to the Doppler effect, a reflected frequency from a moving object will be shifted from the original transmitted frequency. The arterial Doppler signal is formed as follows: an ultrasonic transducer is placed in contact with the skin surface, and the ultrasound beam whose frequency is f.sub.o is directed toward the blood vessel. The source of the echo signals are the red blood cells flowing in the vessel. The reflected signal frequency is denoted as f.sub.r. The Doppler frequency shift, f.sub.D, will be related to the velocity of the reflector, v, by the equation, EQU f.sub.D =(2f.sub.o v/c) cos .THETA. (1)
where c is the speed of sound and .THETA. is the angle between the direction of the blood flow and the ultrasonic beam. It can been seen that f.sub.D will change along with changing .THETA.. When .THETA.=90.degree., cos .THETA.=0, which is an undesirable situation because no Doppler shift will be detected. In practice, the transducer is usually oriented and fixed to make a 30.degree.-60.degree. angle with the arterial lumen to ensure the correct measurement of v.
Since all reflectors flowing at different speeds within the sound beam are contributing to the Doppler frequency shift, f.sub.r, the output f.sub.D is a complex signal occupying a wide frequency range.
Whenever an ultrasound beam strikes an interface formed by two tissues having different acoustic properties, part of the beam is reflected and part is transmitted. If the reflected wave travels back toward the ultrasound source, it may be detected as an echo. The amplitude of the reflected wave is determined by the different in acoustic impedances of the material forming the interface.
The acoustic impedance, Z, is the speed of sound in a material multiplied by its density. For a perpendicular angle of incidence of an ultrasound beam on a large, flat interface, the ratio of the reflected to the incident amplitude, R, is given by EQU R=(Z.sub.2 -Z.sub.1)/(Z.sub.2 +Z.sub.1) (2)
where Z.sub.1 is the impedance of the substance in which the reflected beam will travel, and Z.sub.2 is the impedance of the substance in which the incident beam travels. Equation (2) shows that the larger the difference between Z.sub.1 and Z.sub.2, the more energy is reflected and the greater is the echo.
Despite the existence of Doppler effect techniques used for measuring the acoustic properties of living tissues, a need remains for a simple reproducible, non-invasive technique for detecting when and how much cholesterol has deposited in an individual's arteries.